The present invention relates to a thin film semiconductor device, such as a thin film transistor (TFT) formed on a substrate, such as a transparent substrate, and a method of producing the thin film semiconductor device. More specifically, the invention relates to a new method of forming a semiconductor active layer in such a device.
In recent years, applications of thin film semiconductor devices such as thin film transistors (TFTs) to liquid crystal display devices, three-dimensional circuit devices are studied and developed actively.
In applications of TFTs to liquid crystal display devices, TFTs are incorporated in the respective picture elements (pixels) arranged in a matrix in a liquid crystal display device with a low drive voltage and a small power consumption to obtain a flat display device having a good picture quality. It is expected that the liquid crystal image display device will be wide-spread, replacing CRTs.
FIG. 1 shows an equivalent circuit of one of pixels of a liquid crystal display device. The pixels arranged in a matrix form an image. Each pixel is selected by a gate line 21 for transmitting a scan signal, and a source line 22 for transmitting a video signal. Each pixel comprises a TFT 27 having a gate 25 connected to the gate line 21, a source 23 connected to the source line 22, and a drain 24 connected to a liquid crystal layer 26.
The TFT in such an application is required to have a sufficient large ON current Ion to give a sufficient potential to the liquid crystal layer 26 in a short writing time and a small OFF current Ioff to hold the signal for a period (frame period) of one selection to the subsequent selection.
FIG. 2 shows I.sub.D (drain current)-V.sub.G (gate voltage) characteristic, which is the switching characteristic of the TFT. The characteristic under normal condition is shown by curve 31, with Ioff at 31. When the channel part of the TFT is illuminated by an incident light from the outside, the carriers are excited by the incident light and photo-current flows. The characteristic is therefore as shown by curve 32 with Ioff at 34, higher than 33. With the elevated Ioff, holding the signal for the frame period is difficult and the picture quality of the display becomes poor.
To eliminate the problem, TFTs are provided with an incident light shielding layer.
FIG. 3 shows an example of a conventional TFT. A transparent substrate 1 may be of quartz, on which an incident light shielding layer 2 is formed by, for example, first depositing by sputtering or the like a thin film of chrome or the like and paterning the thin film by photolithography and etching. An insulating layer 3 of silicon oxide film or the like is formed over the entire surface of the incident light shielding layer 2. A polysilicon layer 4 is then deposited over the entire surface by reduced pressure CVD (chemical vapor deposition). The polysilicon layer is then subjected to complicated photolithography, and selective oxidation to be formed into an active layer 6 in an island form. To improve the performance of the TFT, the active layer 6 is re-crystalized by argon laser. FIG. 3 also shows a gate oxide film 8, a gate 9 formed, for instance, of polysilicon, a source electrode 10, a drain electrode 11, oxide film 5, and a gate electrode 12.
To form the TFT with improved performance, the incident light shielding layer 2 must first be formed by photolithography and etching, and then the semiconductor active layer must be formed by even more complicated photolithography and selective oxidation. Moreover, the laser recrystalization process is conducted to improve the performance. Thus the process of fabrication is complicated.